Methods of this kind and apparatuses in similar form have already long been known from the prior art. For instance, it is known to treat plastics material that is to be recycled first at elevated temperature in a cutter compactor and, optionally with exposure to reduced pressure, then to melt it in an extruder and to filter the melt, which is subsequently degassed and, lastly, subjected to pelletizing, for example. Apparatuses for implementing such methods are known for example from EP 123 771 B, EP 390 873 B or AT 396 900 B.
There are also numerous methods and apparatuses in existence for optimizing individual steps, as for example the degassing of the melt. Thus, for example, an unpressurized zone may be provided ahead of the degassing apertures, in order to ensure reliable degassing of the plastics material. There are also numerous embodiments of various melt filters in existence, for the removal of solid extraneous substances and/or unmelted residual polymer.
All of this serves primarily to increase the quality of the end product.
A feature shared by the known cutter compactors and containers mentioned in the introduction is that the direction of conveying or of rotation of the mixing and comminution implements, and therefore the direction in which the particles of material circulate in the receiver (receiving container), and the direction of conveying of the extruder, are in essence identical or have the same sense. This arrangement, selected intentionally, was the result of the desire to maximize stuffing of the material into the screw, or to force-feed the screw. This concept of stuffing the particles into the conveying screw or extruder screw in the direction of conveying of the screw was also very obvious and was in line with the familiar thinking of the person skilled in the art, since it means that the particles do not have to reverse their direction of movement and there is therefore no need to exert any additional force for the change of direction. An objective here, and in further derivative developments, was always to maximize screw fill and to amplify this stuffing effect. By way of example, attempts have also been made to extend the intake region of the extruder in the manner of a cone or to curve the comminution implements in the shape of a sickle, so that these can act like a trowel in feeding the softened material into the screw. Displacement of the extruder, on the inflow side, from a radial position to a tangential position in relation to the container further amplified the stuffing effect, and increased the force with which the plastics material from the circulating implement was conveyed or forced into the extruder.
Apparatuses of this type are in principle capable of functioning, and they operate satisfactorily, although with recurring problems:
By way of example, an effect repeatedly observed with materials with low energy content, e.g. PET fibres or PET foils, or with materials which at a low temperature become sticky or soft, e.g. polylactic acid (PLA) is that when, intentionally, stuffing of the plastics material into the intake region of the extruder, under pressure, is achieved by components moving in the same sense, this leads to premature melting of the material immediately after, or else in, the intake region of the extruder. This firstly reduces the conveying effect of the extruder, and secondly there can also be some reverse flow of this melt into the region of the cutter compactor or receiver, with the result that flakes that have not yet melted adhere to the melt, and in turn the melt thus cools and to some extent solidifies, with resultant formation of a clump or conglomerate made of to some extent solidified melt and of solid plastics particles. This causes blockage on the intake of the extruder and caking of the mixing and comminution implements. A further consequence is reduction of the throughput of the extruder, since adequate filling of the screw is no longer achieved. Another possibility here is that movement of the mixing and comminution implements is prevented. In such cases, the system normally has to be shut down and thoroughly cleaned.
Problems also occur with polymer materials which have already been heated in the cutter compactor up to the vicinity of their melting range. If overfilling of the intake region occurs here, the material melts and intake is impaired.
Problems are also encountered with fibrous materials that are mostly orientated and linear, with a certain amount of longitudinal elongation and low thickness or stiffness, for example plastics foils cut into strips. A main reason for this is that the elongate material is retained at the outflow end of the intake aperture of the screw, where one end of the strip protrudes into the receiver and the other end protrudes into the intake region. Since the mixing implements and the screw are moving in the same sense or exert the same conveying-direction component and pressure component on the material, both ends of the strip are subjected to tension and pressure in the same direction, and release of the strip becomes impossible. This in turn leads to accumulation of the material in the said region, to a narrowing of the cross section of the intake aperture, and to poorer intake performance and, as a further consequence, to reduced throughput. The increased feed pressure in this region can moreover cause melting, and this in turn causes the problems mentioned in the introduction.